Particle therapy systems for treatment of patients are known. In particle therapy, charged particles such as protons or heavy ions are used to irradiate a target volume such as tumor. One unique feature of particle therapy is that charged particles release most of their energy around the area where they stop because of the “Bragg peak” effect. Therefore, by controlling the energy of charged particles, healthy tissue distal to the source of charged particles receives substantially no radiation and healthy tissue proximal to the source receives significantly reduced amount of radiation. Furthermore, by choosing the energy of charged particles for irradiating different “layers” or “depths” of the target volume, radiation dose distribution can be tailored to the shape of the target in all three dimensions.
Movement of a patient undergoing particle therapy presents a challenge, including the accuracy with which the target will be irradiated and sparing the healthy tissue. Normal physiological movement such as respiration or cardiac cycles can cause positional changes of the target volume during treatment. To irradiate a target that is not stationary, “gating” is normally utilized to block the radiation beam whenever the target is out of position. In a gating approach, instruments are utilized to measure the physiological state and/or movement of the patient. These instruments can produce a signal indicative of the movement of the target e.g. during the respiratory cycle. The radiation beam can be gated based upon certain threshold amplitude levels of the measured signal such that the radiation beam is blocked or disengaged during particular time points in the signal that corresponds to excessive movement of the target. FIG. 1 illustrates a conventional particle system 1 where a gating system 2 is coupled to a beam delivery system 3. Gating signals 4, which are synchronized to the movement of a target, are provided to the beam delivery system 3 for turning on and off the radiation beam.
While the conventional gating approach can reduce the amount of healthy tissue irradiated, it typically requires that the beam be turned off for a large period of time during the treatment, leading to increased treatment time and decreased patient throughput. The gating approach also represents inefficient usage of the facility's most valuable resource, beam time.